Recently, inkjet printers have been rapidly diffused for its advantageous properties such as low noise and low running cost and color printers which are printable on the regular paper have been actively put into the market. However, satisfying all the properties required such as color reproducibility, endurance, light stability, drying property, feathering and color bleed of images, duplex printing ability, discharge stability, and the like is very difficult and inks are therefore selected depending on the preferred but property corresponding to the purpose.
The inks used for inkjet printers in general consist mainly of water and contain colorant and humectant such as glycerin, and the like for anticlogging. Dyes are used as colorant for its excellent color-producing property and stability. However, light stability and waterproofness, etc. of the images produced by dye-based inks are not satisfactory. Waterproofness may be improved somehow by upgrading the inkjet recording paper containing ink absorber layer, however, it is unsatisfactory for the regular paper.
Recently, a pigment ink in which a pigment such as organic pigment, carbon black, and the like instead of dyes is dispersed in water or an aqueous medium is being investigated for use. Examples of general method for producing pigment dispersion ink include high speed agitation by homogenizers, kneading-dispersion machines using balls such as bead mill or ball mill, kneading-dispersion machines using shear force such as roll mill, a method for preparing dispersion liquid using ultrasound dispersion machines, etc. after dissolving a dispersant in an aqueous medium and sufficiently moistening by adding pigment particles. For example, a method for improving discharge stability in which an organic pigment is reduced to particles of 50 nm or less in diameter using specific dispersant is disclosed in Patent Literatures 2 and 3. And an ink for inkjet printing using specific organic pigment and anionic dispersant is disclosed in Patent Literature 4.
Recording inks for inkjet printing with pigment particles of 50 nm to 150 nm in the volume average particle diameter are being used in general. If the particle is larger than this size, it causes clogging of nozzles and if it is smaller, storage stability is significantly deteriorated. There are various dispersants from surfactants of low molecular mass to polymer dispersants of high molecular mass. A polymer dispersant excels in storage stability; however, it is difficult to reduce the volume average particle diameter of a pigment particle to less than 150 nm. On the other hand, with a dispersant of low molecular mass, it is possible to reduce the volume average particle diameter of a pigment particle to 50 nm to 100 nm; however, storage stability may not be satisfactory. Methods for adding water-soluble polymers are proposed in Patent Literatures 1 and 6 as a method to compensate for the lack of storage stability of dispersant of low molecular mass.
And others include surface reforming pigment ink in which the surface of the pigment particles are modified with carboxyl group, carbonyl group, sulfone group and hydroxyl group, and the like to give hydrophilic property for stable dispersion. A surface reforming carbon black, a black-based pigment ink in which pigment particles can be stably dispersed by introducing hydrophilic groups to the surface of carbon black, has been developed and put in practical use. A color pigment which can be stably dispersed has also been developed for color pigment inks. However, these surface reforming pigment inks have disadvantages of insufficient scratch resistance and waterproofness on the regular paper and the exclusive gloss paper.
An inkjet ink utilizing a dispersing element of micro-capsule or emulsion type by which the pigment particles are coated with resin has been proposed. Since the pigment particles are coated solidly by resin, dispersion may be stable for a long period and the discharge stability may improve somewhat, however, it is difficult to produce a dispersion particle of small particle diameter suitable for inkjet printing and there is a problem of high cost because of complicated production process.
For example, an approach of adding fluorochemical surfactant to the ink and letting it spread broadly and uniformly in a concentric fashion after an ink droplet is applied on the paper to improve filling of the spaces between dots of ink droplets and saturation in order to improve saturation degradation of printed images caused by large diameter of dispersion particles, has been proposed in Patent Literatures 7 and 8.
When printing in red color with magenta and yellow inks, a phenomenon may be observed in that the color tone differs depending on the order of ink applied on the paper. The phenomenon becomes prominent in the inkjet method in which printing is performed by scanning an ink discharge head because application order is reversed between outward and homeward path and it is not acceptable especially in a region of high concentration of pigment inks. Similarly, when printing in green color with yellow and cyan inks or when printing in blue color with cyan and magenta inks, bidirectional color difference may also occur. This has been handled by setting a limitation on the bidirectional printing making the printing speed to slow down.
Therefore in the Patent Literature 4, it is stated that the bidirectional color difference can be decreased by utilizing an ink having at least four different hue as a color ink. However, red, green and blue inks are not used in this proposal and it is of different ink set formation.
Moreover, when secondary colors of red, green and blue are formed by using yellow, magenta and cyan inks, the text looks blurred because all the dots are not filled and spaces are left at the tone exceeding total amount control. When yellow ink is used for the secondary color, the text still looks blurred because of the low visibility of yellow color and the text quality is deteriorated. Furthermore, when secondary color is formed, two kinds of inks are applied in the same dot to form one tone and double amount of total application is needed to express the same tone as a single color ink. However, because the tone exceeding total amount control cannot be expressed, the number of tone is lessened and therefore not favorable.
A red ink can be produced by simply mixing magenta and yellow inks. Bidirectional color difference, text quality and tone property can be improved by utilizing the mixed red ink better than utilizing two different colors. However, there is no difference in saturation between the mixed red ink and two different colors; therefore it is impossible to expand the region of color reproduction.
Moreover, further improvement of saturation is possible by utilizing single red pigment. Particularly, the red ink of high saturation can be prepared by diketopyrrolopyrrole red pigment (Patent Literature 5). Similarly, it is possible to achieve a color tone of high saturation for green inks by utilizing single phthalocyanine green pigment better than applying two colors. However, there is an issue of tone gradually moving off to the yellow side as the image density and saturation are increased when utilizing an ink of diketopyrrolopyrrole red pigment. This phenomenon becomes distinguishable around 90% ink duty and the hue transition becomes greater in a region of high density and saturation.
It is difficult to perform a stable dispersion in an aqueous medium when single red pigment, green pigment and in particular diketopyrrolopyrrole red pigment and phthalocyanine green pigment are utilized. When they are employed as inks for inkjet printing, particle diameter must be as small as approximately 100 nm or less and it is necessary to select based on the compatibility with dispersant and pigment or with additive water-soluble solvent in order to retain the ink of small particle diameter under stable condition for extended period.
Therefore, the recording ink and ink set in which diketopyrrolopyrrole red pigment and phthalocyanin green pigment with excellent color development can be stably dispersed in an aqueous medium; properties remain the same after long storage; and are suitable as red and green inks with appropriate discharge stability by inkjet have not been made available in the present situation.    [Patent Literature 1] Japanese Patent (JP-B) No. 3625595 (Japanese Patent Application Laid-Open (JP-A) No. 09-263720)    [Patent Literature 2] JP-A No. 09-263722    [Patent Literature 3] JP-A No. 2002-088286    [Patent Literature 4] JP-A No. 2003-276300    [Patent Literature 5] JP-A No. 08-239610    [Patent Literature 6] JP-B No. 3618493    [Patent Literature 7] JP-A No. 2003-268274    [Patent Literature 8] JP-A No. 2003-335987